This disclosure relates to an apparatus and method for processing an analog signal into a digital signal, for example, in an image sensor.
An image sensor may comprise an array of photodetectors for imaging light focused onto the image sensor. Each photodetector may include a photodiode or phototransistor which generates charge in proportion to the intensity of light incident on the photodetector. The charge generated by the photodetector is stored in a charge well, e.g., on a capacitor. A clock synchronizes and controls the readout of the stored packets of charge from the charge wells, and an amplifier converts each packet of charge to a voltage. An analog-to-digital converter (ADC) further converts the analog value of each voltage into a corresponding digital value.
The ADC may be provided at chip-level, column-level, or pixel-level. A chip-level ADC performs analog-to-digital (A/D) conversion for all photodetectors in the image sensor array, a column-level ADC performs A/D conversion for the photodetectors in a single column or group of columns, and a pixel-level ADC performs A/D conversion for a single photodetector. Increasing the number of ADC's can increase the speed with which the A/D conversion is performed. However, providing more ADC's can result in an increase in the complexity and footprint of the necessary A/D conversion circuitry.
ADC's are conventionally implemented as complementary metal-oxide semiconductor (CMOS) devices. In contrast, image sensors may be implemented as charge coupled devices (CCD's) or CMOS devices. In a CCD, the packets of charge generated by each photodetector are transported through successive charge wells, i.e. in a bucket-brigade arrangement. A vertical shift register shifts the charge stored in each row in the image sensor array downward towards a horizontal shift register. The horizontal shift register then shifts out the charge stored in each successive row towards an amplifier. Typically, A/D conversion of CCD image data is performed off-chip, i.e. at chip-level, since the semiconductor processing required for conventional CMOS ADC's and CCD's is incompatible. In contrast, CMOS image sensors can more readily incorporate additional circuitry, e.g. pixel-level CMOS ADC's.